A difficulty regarding crankcase-scavenged engines is to provide a homogeneous air-fuel mixture to the combustion chamber, especially if the engine is provided with additional air supply to the transfer ducts. A homogenous mixture can be achieved by so called long transfer ducts, which however tends to make the crankcase complicated and bulky. For two-stroke engines provided with additional air to the transfer ducts it is important to keep the air in the transfer ducts separated from the air-fuel mixture, in order to as far as possible prevent the air-fuel mixture from the transfer ducts to disappear out through the exhaust port. This separation, also called stratification, is often promoted by making the transfer ducts long and narrow, thus preventing, or at least reducing, mixing of different scavenging gases. The length is also adapted to the desired performance of the tool and its engine. Long transfer ducts for high torque at low speed and shorter ducts for high torque at high speed.
However, there is a tendency that speed dependent pressure variations are created in the transfer ducts of the engine during operation. These pressure variations are caused by oscillation of the gases contained in the transfer ducts. These pressure variations are particularly big for long and narrow transfer ducts, but they can also be fairly big also for short and narrow transfer ducts. These pressure variations change with the speed of the engine. When opening the supply of additional air to the transfer ducts at different speeds this would lead to reduced feed of air at some speeds and increased air feed at other speeds. Therefore the operation of the engine is not as good as intended. The variations in the amount of supplied additional air to the transfer ducts leads to a variation with speed in the overall air fuel ratio of the engine, and is therefore a problem.